A magnetic resonance imaging (hereinafter, referred to as “MRI”) apparatus is an apparatus which measures a nuclear magnetic resonance (hereinafter, referred to as “NMR”) signal generated by atomic nucleus spins forming an object, especially, tissues of a human body, and generates morphologies or functions of the head, the abdomen, the limbs and the like of the human body as two-dimensional or three-dimensional images on the basis of a nuclear density distribution, a relaxation time distribution or the like.
In MR imaging, an NMR signal is added with different phase encodes and frequency encodes due to a gradient magnetic field, and is measured as time-series data. The measured NMR signal is subjected to two-dimensional or three-dimensional Fourier transform so as to be reconstructed as an image.
In a high magnetic field MRI apparatus, a resonance frequency of proton is high, and thus a wavelength of a high frequency (hereinafter, referred to as “RF”) pulse which is applied to an object is shortened. Thus, a spatial distribution (B1 distribution) of an irradiation RF pulse is non-homogeneous, and thus a problem such as shading or a reduction in an SNR occurs. If a frequency of the irradiation RF pulse is high, an SAR indicating an absorption amount of the RF pulse absorbed by a living body tissue of an object is also high.
As a method of improving the B1 non-homogeneity, there is RF shimming disclosed in PTL 1. The RF shimming method is a technique in which shimming parameters having different amplitude ratios and phase differences are respectively set for a plurality of channels of RF irradiation coils (hereinafter, referred to as irradiation channels), and a spatial distribution of an irradiation RF pulse is made homogeneous.